I commonly hear that compound exercises are the best mass builders, but I just can not understand why. Yes, they can save time by doing more at the same time. I also realize that they involve stabilizing muscles, which can be useful for building basic strength for newbies and they are also much less likely to get you disproportionaly strong in one muscle group, but all of that has very little to do with being great specifically for building mass.

The common argument is that they let you lift much heavier, but if you disperse the load among all the muscles involved, you will likely end up with similar loads per muscle group as in an isolation exercise. Actually I would tend to believe that doing more groups at once will probably result in a much less controlled movement, and the secondary muscle groups involved could be a limiting factor in squeezing the most out of your targeted group.

If we take chin ups as an example, you would not be taking your lats nor your biceps to the limit at all. Would it not be better to do curls and a seated rowing machine instead?

For me it is not just a question of exercising the hell out of the muscle I want, but a question of leaving that muscle to rest for a while, and no matter how I try to structure my workouts, having compound exercises will always overlap and the muscles that should be resting will be getting punished unintentionally on a different muscle group day.

Maybe I am completely missing something, but by this logic I just can't see how compounds are that great for mass. I would appreciate if someone could explain.

Who says compound movements are best for mass gains? Bodybuilders certainly isolate muscles and they know how to gain muscle mass better than anyone. It's likely true that deadlift and squat give you a hormonal response that isolation doesn't but it doesn't have to be one or the other.
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michaelSep 12 '12 at 15:33

Where did somebody say that compound exercises are the best mass builders? Did they qualify their statement to say under what conditions they are the best? Are you assuming the claim is universal, for everyone, at all times? I've never heard such a claim. You basically debunked the claim in your own question, so, what is your question?
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user4244Sep 12 '12 at 16:08

2 Answers
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All things have to be taken in context, and understanding the target audience. Some compound movements like Squats and Deadlifts have a great anabolic effect, which puts the body in a place where it is more likely to put on muscle. Particularly beginners are best served focusing on compounds for both the strength and size considerations. However, that is not the end of the story, nor can it be.

Goals of Your Training

Your goals affect the type of training. They are paramount, as they help you determine what kind of a program and what exercises are necessary to achieve those goals. It also affects what type of results you want.

If your goal is to have large masses of muscle, you train for size

If your goal is to perform XYZ, you train for the technique, strength, and mobility required to do XYZ

Muscles can grow in a couple ways, both are hypertrophy, but the nature of the hypertrophy is different:

Sarcoplasmic hypertrophy is an increase in the energy support systems, and is the type of muscle growth sought out by bodybuilders. Muscles are large and spongy.

Myofibrilar hypertrophy is an increase in the protein pairs that perform contraction, and is the type of muscle growth sought out by strength athletes (power lifting, olympic lifting, strong man). Muscles are dense and hard--i.e. not as large.

Most athletes do need a combination of both of these, but depending on your goals one may take priority over the other.

What causes growth?

This is a very valid question, and affects whether compounds or isolations are necessary for your goal at hand. There are some fundamental things that must be balanced in your training that work for everyone:

Fatigue--you can use fatigue as a training variable (i.e. reducing rest times or training to failure)

Rest--all growth occurs during recovery

Essentially, there is a balance of training stress and recovery that has to happen. The stress puts the muscles in a state where they will have priority when the body repairs itself. The recovery is where the actual growth occurs.

Compound lifts stress more of your body at once. However, this requires that you take your training and rest as complete days on or off. This approach seems to work well for strength sports.

Isolation lifts focus the stress on one part of your body. This allows you to really stress that part of your body one day, and cycle to other parts of your body while the stressed one recovers.

Parting Thoughts

This is not entirely an either/or proposition. These are simply training options. Some people do well with mixing approaches. For example you could use squats, deadlifts, and bench press to elicit its general anabolic effects; but then focus on smaller body parts in isolation.

It would be a mistake to completely ignore either compounds or isolation work. Particularly since some isolation work can address weaknesses we may have in part of the kinetic chain of muscles. For example, if you have a strength imbalance from the left and right side, you need to exercise them independently to make the weaker side stronger.

Berin's answer is a good one. Total volume is something that you should consider. For eg. If you are training your body with a lot of isolation work than your lifts on the compound exercises are going to come down. Heavier weights is what will increase your muscle mass. Light weights with more reps doesn't increase size but causes muscles to retain more fluid which gives a perception of growth. This growth quickly vanishes when your workouts or diet goes missing for some time. I would also recommend you to read a book by Christian Thibadeau called "Black book of training secrets", he explains this concept well.